The embodiments described herein relate generally to gas turbine combustion systems and, more particularly, to fuel and air premixers that facilitate reducing damage during an off-design flame holding event.
At least some known gas turbine engines ignite a fuel-air mixture in a combustor to generate a combustion gas stream that is channeled to a turbine via a hot gas path. Compressed air is delivered to the combustor from a compressor. Known combustor assemblies include fuel nozzles that facilitate fuel and air delivery to a combustion region of the combustor. The turbine converts the thermal energy of the combustion gas stream to mechanical energy used to rotate a turbine shaft. The output of the turbine may be used to power a machine, for example, an electric generator or a pump.
Emissions produced by gas turbines burning conventional hydrocarbon fuels may include oxides of nitrogen, carbon monoxide, and unburned hydrocarbons. It is well known in the art that the oxidation of molecular nitrogen (NOx) in air breathing engines is dependent upon the hot gas temperatures created in the combustion system reaction zone. One method of reducing NOx emissions is to maintain the temperature of the reaction zone of a heat engine combustor at or below the level at which thermal NOx is formed by premixing fuel and air to a lean mixture prior to the mixture being ignited. Often such a process is done in a Dry Low NOx (DLN) combustion system. In such systems, the thermal mass of excess air present in the reaction zone of the combustor absorbs heat to reduce the temperature rise of the products of combustion to a level where the generation of thermal NOx is reduced.
During the combustion of gaseous or liquid fuels, known lean-premixed combustors may experience flame holding or flashback in which the flame that is intended to be confined within the combustion liner travels upstream towards the injection locations of fuel and air into the premixing section. Such flame holding/flashback events may result in degradation of emissions performance and/or overheating and damage to the premixing section, due to the extremely large thermal load. At least some known gas turbine combustion systems include premixing injectors that premix fuel and compressed airflow in an attempt to channel uniform lean fuel-air premixtures to a combustion liner. Typically, a bulk burner tube velocity exists, above which a flame in the premixer will be pushed out to a primary burning zone.
As more reactive fuels, such as synthetic gas (“syngas”), syngas with pre-combustion carbon-capture (which results in a high-hydrogen fuel), and/or natural gas with elevated percentages of higher-hydrocarbons are used, current DLN combustion systems may have difficulty in maintaining flame holding during engine operation. In ideal operating conditions, a flame inside the premixer does not remain in the premixer, but rather is displaced downstream into the normal combustion zone. Since the design point of state-of-the-art combustion systems may reach bulk flame temperatures of 3000° F., flame holding/flashback events may cause extensive damage to the premixing nozzle section in a very short period of time.